Turn the "closed" property on cluster repositories into a nice boolean

Summary:
Ref T10883. Ref T13120. There's an existing "closed" property on repository services that stops new repositories from being allocated there.

Turn it into a nice boolean.

Test Plan: Toggled the value on/off using a nice <select /> with helpful labels instead of a text area.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13120, T10883

Differential Revision: https://secure.phabricator.com/D19355